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Fluid samples show the range of appearances of used fluid in various ages and conditions. |
As a sales engineer working with heat transfer fluids in the process industries for the past eight years, there is one question that I get asked over and over and over again, sometimes two to three times per week: “How long will my thermal fluids last?” Though the question comes in many forms - “What is the life expectancy of your brand?” “Is your fluid going to last longer than brand X or brand Y?” “How many hours of run time before I change?” - the one thing we know for sure is the fluid will not last forever.
Fluid degradation over time is inevitable. The goal is to minimize the rate at which it happens. No matter the brand of thermal fluid, the life of the thermal oil depends on several factors such as temperature, flow and production schedule. But, it but mostly depends on preventive maintenance.
To demonstrate this, suppose you could study three identical processes, using the same heat transfer fluid, in three different plants, all operating with the same temperatures, flows and production schedules. Now, everything is the same, right? Yet the same fluid brand in one setup might last a year, another might last three years and the third could last five years or more. The deciding factor in most cases is preventive maintenance. Preventive maintenance can reduce the effect of oxidation and thermal cracking. It can also decrease risks such as injury or fire.
The purpose of this article is to get you thinking about a thermal fluid preventive maintenance program. Consider this question: If the heat transfer fluid is vital to your production, and the production is your lifeline to profit, why is the thermal fluid the last thing you think of?
In many cases, the following preventive maintenance suggestions do not cost the company any money and may end up saving money, preventing downtime and increasing peace of mind. These 10 tips for extending fluid life should help keep your thermal fluid system running and maximize fluid life.
1. Pick the Right Fluid
Use a quality thermal fluid designed for your operating temperature and other conditions. General-purpose lubricants and hydraulic oils will not provide the same life or efficiency as a dedicated thermal fluid. General-purpose products and hydraulic oils often contain lubricant-performance additives that can cause trouble at high temperatures.
2. Perform Fluid Analysis
The best way to tell whether your system is running properly or if there are any system-related red flags is to have your thermal fluid supplier test your fluid. A quality supplier should be able to test the fluid, analyze the results and recommend corrective action.
3. Know How to Start Up and Shut Down
One area where your production schedule may have a significant influence on fluid life is startups and shutdowns. Stops and starts can be hard on the fluid and the equipment, especially if done carelessly.
When starting the process, bring the temperature of your heater up slowly until about 180°F (82°C). This is the average temperature at which most fluids reach turbulent flow. In turbulent flow, there is a smaller difference between film temperature and bulk temperature, which means as the temperature increases, there is less thermal stress on the fluid molecules. Once you reach turbulent flow, you can dial up to your process temperature.
When shutting down, turn off the heat to the process but keep the pump running. Once the temperature reaches 200°F (93°C), shut the pump down.
4. Use a Warmup Valve
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Preventive maintenance can keep your thermal-fluid system running and maximize fluid life. This is an image of a thermal oil heater manufactured by Factory Sales and Engineering. |
Always run with the warmup valve or boil-off line closed - once you have flashed off your low boilers, including Paste water.
5. Keep the Expansion Tank Cool
Make sure your expansion tank remains cool to the touch. If your expansion tank runs hot because of the design of the system, a nitrogen blanket at about 1 or 2 psi will reduce oxidation. Never insulate the expansion tank or the expansion line going to the tank.
6. Check the Low-Level Switch
Is the low-level switch operating? On some units, this mechanical device will get stuck in the closed position. If the process is running on low oil, the oil will deteriorate rapidly. If you have a sight glass, make sure it is clean and you can see the true level. If you have dip tubes, make sure they are not plugged.
7. Check Your Stack Temperature
Watching stack temperature is a great way to tell if your fluid is coking. Higher stack temperatures mean the heat is not transferring to the oil. Instead, it is going up the chimney. Check and log the stack temperature on a monthly basis.
8. Ensure Good Flow
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Fluid degradation over time is inevitable. The goal is to minimize the rate at which it happens.
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Check the pressure differential across the heater against the thermal fluid heater manufacturer’s specifications. If it does not check out, a low flow rate could be degrading the fluid.
9. Store Heat Transfer Fluid Drums Properly
Keep your thermal fluid drums dry and clearly label used thermal fluid drums. This helps with limiting contamination, which can shorten fluid life drastically.
10. Invest in Training
Make sure all of the operators and maintenance personnel are trained properly. Follow operation and maintenance procedures in the heater and heated equipment manufacturers’ manuals.
Thermal fluid allows you to utilize high temperatures at low system pressures. It offers a safe and extended equipment operating life. It allows for efficient, noncorrosive and accurate temperature regulation. But like any piece of process equipment, it requires preventive maintenance. Consider printing out these guidelines, laminating them and taping them to your process. Then, walk your thermal fluid system monthly. You can learn a lot about your system by listening and looking. PH
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